Started on the 25th April they wrote "At 13:54 Stickers on the Central Line took its first steps. After months (well hours) of planning we were finally ready. Armed with stickers we boarded the 13:50 from Epping to West Ruislip and unleashed phase 1 onto an unsuspecting public."
The stickers are pretty inventive and each one comes with a commentary on the blog. I love the explanation for Lille: "It is a little known fact that between Woodford and Buckhurst Hill there is a lightly used tunnel which provides London Underground commuters direct access to the Paris Metro. Providing you’re not in a hurry, you can just sit back (if you can find a seat), relax and simply show your passport when the border guards pass through the train."
As well as the blog, there's also a Facebook Group where you can record your spots and send in ideas for stickers.
Thanks to the many people who let me know about this and hat-tip to @Londonist on Twitter where I first saw it mentioned.